SUPER EXTENDED MEMORY SUPPORT

These changes are intended to provide support for extended memory pools
up to 4 Gb in size. The current XMS API, since it uses 16-bit values to
specify block sizes in Kb, is limited to 64 Mb maximum block size.
Future machines are expected to support memory above 64 MB.

This support is implemented in the form of extensions to existing
functions, rather than entirely new entry points, to allow for more
efficient implementations.

Programs should generally use the existing functions, instead of these
extended ones, unless they have an explicit need to deal with memory
above 64 Mb.